Imaging apparatus



Sept. 29, 1959 FIG.

w. GLASER IMAGING APPARATUS Filed July 25, 1956 FIG.3

FIG. 4

I B2 I I I I I I l I I I I I l I I I l I I I I I I I z 2 2| Z2 INVENTOR Walter Glaser BY ATTORNEYS United States Patent IlVIAGING APPARATUS Walter Glaser, Mount Vernon, N.Y., assignor to Farrand Optical C0., Inc., New York, N.Y., a corporation of New York Application July 25, 1956, Serial No. 600,136

8 Claims. (Cl. 250--49.5)

This invention relates to image-forming devices and more particularly to lenses adapted to form images by means of charged particles. The invention provides a lens of this type having reduced spherical aberration.

The invention will now be further described with reference to the accompanying drawings in which Figs. 1 and 2 are respectively axial sections through two forms of lens according to the invention; and

Figs. 3 and 4 are graphs useful in explaining the invention.

As is well known, rotationally symmetric fields have, at least within the paraxial region, image-forming properties, so that charged particles diverging from one point adjacent the axis of the field with a component of axial velocity will be imaged or brought to focus at another point displaced along the axis. For aperture angles larger than those corresponding to paraxial image formation however, the various rotationally symmetric fields are not equally favorable for image formation. The lenses now widely used in electron microscopes which, in the magnetic case for example, include essentially a coil positioned coaxially about the intended axis of image formation, produce a field which is afflicted with spherical aberration, so that object points located on one side of the plane of the coil are imaged with spherical aberration at locations on the opposite side of the plane of the coil.

Rotationally symmetric fields may be identified by the function describing the axial component thereof at successive points along the axis of symmetry, the potential theory making it possible to predict from the form of this component the nature of the extra-axial field, as to both radial and axial components. For a magnetic electron lens of the known type of the prior art above described, this component has a bell shape, as indicated in Fig. 4.

The spherical error in an image-forming rotationally symmetric field can be written as In this equation the z-axis of coordinates is the symmetry axis of the field. p is the diameter of the aberration disk in an image plane perpendicular to the system axis of z-coordinate z produced by a point object in a similar plane of z-coordinate Z1. is the aperture angle,

is the specific charge of electron, U is the acceleration potential, 3 is the axial component of the field at points along the z-axisand h is an electron trajectory equal to wherein r is the radial position of the electron and r the radial position the electron at the axial position z=z of the aperture-limiting diaphragm.

For the integrand to be zero, it is clear that B B must be positive so that for positive values of B,,- B,"

must also be positive. Hence the curve plotting as ordinate the axial component B of the field at points along the axis as a function of the axial coordinate must be concave upward. This condition is evidently fulfilled for only a part of the bell shaped field of Fig. 4.

In fact, setting the integrand equal to zero in the above equation gives'a differential equation corresponding to zero aberration error, and the solution of this differential equation specifies a field varying in the manner shown in Fig. 3. In Fig. 3 the ordinate B is the axial component, at points along the axis, of the field specified by the solution to the differential equation above described, as a function of position along the z-axis which is the symmetry axis of the field. The field strength plotted in Fig. 3 increases to infinity at two axial positions. Consequently only a part of the field of Fig. 3 can be realized in practice.

It has been heretofore proposed to produce a field ap proaching generally the shape of Fig. 3, with modifications at the ends thereof, of course. heretofore proposed have however produced a field more nearly V-shaped than the U-shape specified in Fig. 3. The present invention provides lenses developing between axially displaced positions a trough-shaped field closely approximating that of Fig. 3. Object and image position defining means are then provided within the troughshaped portion of the field which is continuously concave upward between these positions for positive values of B Figs. 1 and 2 illustrate two lenses according to the invention approximating the desired form of field for zero spherical aberration. In Fig. 1 two circular coils 2 and 4 are shown supported, by means not shown, coaxially in an axis 2. Both of the coils are armored with an iron casing 6 having a circumferential gap 8 of short axial extension on the radially inner surface of the coil. These casings steepen the shape of the individual bellshaped fields associated with each of the coils 2 and 4, and the coils are spaced so that the two bell-shaped fields combine to provide a field whose axial component along the axis varies with axial position as indicated by the dash line curve 10. Typical object and image positions are indicated in Fig. 1 at Z1 and 22 which, it will be noted, lie within the trough-shaped portion of the field identified by the curve 10. A suitable object-supporting device 12, such as a stage or diaphragm, may be provided at Z1, and a suitable image-receiving or particle-recording device 14 may be provided at 2 In view of the location z 'and Z2 with respect to the field 10, an object positioned at Z1 can be imaged at 2, with less spherical aberration than is achievable with the usual lens producing a bellshaped field of the type illustrated in Fig. 4. The imagerecording device 14 may take the form of a photographic plate, fluorescent screen or particle-counting device.

Because of the existence of the field to the left of the position Z1 and to the right of the position Z2, the lens of Fig. l is not expected to be operated in cascade with other lenses either before or after.

A preferred embodiment of the lens of the invention is shown in Fig. 2. This comprises a cylindrical current-- carrying coil 20 iron shielded on the outside by means of a casing 22 having end walls 24 and 26 and iron shielded on its radially inner surface by means of a cylindrical member 28 except for narrow slits adjacent the ends of the coil 20 as indicated at 30. Energization of the coil 20 by means of a suitable direct current produces the effect of two coaxial annular magnetic fields which combine to produce a trough-shaped field similar to the field of Fig. 3 as indicated in Fig. 2 by the dash line curve 31. Provision of the inner cylindrical iron shield member 28 causes the trough-shaped field 31 to decline very rapidly with departure of position from either of the The structures end-walls 24 and 26 and approach toward the mid-plane of the lens so that the field 1&1 constitutes a good approximation to the field of Fig. 3. The object position .z may be immediately inside one of the end walls of the casing, e.g. adjacent the .end wall 24 which may be apertured as at 32 "for suitable illumination of the object if the object is not self-luminous with charged particles such as electrons. The image'location Z2 will then be adjacent the opposite end of the casing where an image-recording device may be'located as described 'in connection with Fig. 1.

While compounding or cascading .of lenses according to the invention does not presently appear to be advantageous, high ultimate magnifications canibe produced for example by recording the image at the image location Z2 on a fine-grained photographic plate which can thereafter be examined by means of a light microscope.

I claim:

1. A lens adapted to form images by means of charged particles comprising means to generate two contiguous coaxial fields the axial .component of each of which at points along the axis thereof varies according to .a bellshaped function, and object and image plane defining means located between the maxima of said fields.

2. Alens adapted to form images by means of charged particles comprising means to generate two contiguous coaxial magnetic fields the axial component of each .of which at points along the axis thereof varies according to a bell-shaped function, and object andirnageplanedefining means located betweenthe maxima of said wfields.

3. A lens adapted to form'images by means of charged particles comprising two annular coils, means to support said coils coaxially, and means defining image and object planes transverse of said axis between said coils.

4. A lens adapted to form images by meansof charged particles comprising two annular coils, means to sup:

port said coils coaxially at such a spacing that the bell-' shaped fields thereof overlap, and means defining image and object planes transverse of said axis between said coils.

5. A lens adapted to form images by means of charged particles comprising two annular coils, a soft iron casing enclosing each of said coils except at a portion of the inner surface thereof, means to support said coils coaxially, and means defining image and object planes transverse of said axis between said coils.

6. A lens adapted to form images by means of charged particles comprising two annular coils, a soft iron casing enclosing .each of said coils except at a portion of the inner surface thereof, means to support :sa'id coils coaxially at such a spacing that the bell shaped fields thereof overlap, and means defining image and object planes transverse of said axis between said coils.

7. A lens adapted to form images by means of charged particles, said lens comprising a cylindrical ferromagnetic casing, ferromagnetic end walls arranged on the casing extending substantially across the axis of said casing, a cylindrical coil inside the casing reaching substantially from end to end thereof, and a ferromagnetic cylindrical member inside the coil positioned coaxially thereof, .said member extending continuously between said end walls except for a narrow annular gap adjacent each of said end walls.

8. A lens adapted to form images by means of charged particles comprising :means to generate a rotationally symmetric field whose component parallel to the axis of symmetry at points along said axis has a generally troughlike shape, said means comprising a current-carrying coil of greater length than diameter, an external ferromagnetic shield substantially enclosing said coil, and a cylindrical ferromagnetic member disposed within said coil and extending from said shield at one end of said coil to said shield at the other end of said coil except for a narrow annular gap at each end of said member.

References Cited in the file of this patent UNITED STATES PATENTS 2,131,185 Knoll Sept. 27, 1938 2,219,113 Ploke Oct. 22, 1940 2,586,559 Page Feb. 19, 1952 

